International Publication No. 2004/092794 (Patent Literature 1) discloses an optical fiber including a core (with a radius r1 and a relative refractive index difference Δ1) extending in the axial direction while containing the axial center of the fiber, a first optical cladding layer (with a radius r2 and a relative refractive index difference Δ2) surrounding the core, a second optical cladding layer (with a radius r3 and a relative refractive index difference Δ3) surrounding the first optical cladding layer, and a jacket layer surrounding the second optical cladding layer. The relative refractive index differences Δ1 to Δ3 satisfy a relationship of Δ1>Δ2>Δ3. Such a refractive index structure is called trench structure. A trench optical fiber has a small bending loss and has been suitably used as an optical transmission line for fiber to the x (FTTx) in recent years, mainly for intra-office wiring and the like in which optical fibers tend to be bent.
Patent Literature 1 discloses as follows. In the trench optical fiber, setting the ratio (r1/r2) of the radius r1 of the core to the radius r2 of the first optical cladding layer to 0.22 to 0.4 realizes a mode field diameter whose ratio to the mode field diameter of a general-purpose single-mode optical fiber having the same cutoff wavelength is 0.98 or larger. This reduces the splicing loss occurring in the fusion-splicing of the trench optical fiber to the general-purpose single-mode optical fiber. The general-purpose single-mode optical fiber referred to herein is an optical fiber not including the second optical cladding layer and having a simple, substantially stepped refractive index profile.
In recent years, the field of use of trench optical fibers, such as the one disclosed by Patent Literature 1, has expanded. In some cases, trench optical fibers are used as optical transmission lines for relatively long-distance optical transmission. Optical transmission lines must have a sufficient level of an optical signal-to-noise ratio (OSNR) so that transmission errors do not occur. The trench optical fiber disclosed by Patent Literature 1, however, has a relatively large transmission loss, making it difficult to provide a sufficient level of OSNR.